The present invention relates to a novel organosilicon compound having two polysulfide linkages in a molecule not known in the prior art and not described in any literatures and a method for the preparation thereof as well as a rubber composition comprising the novel organosilicon compound. More particularly, the invention relates to the above mentioned novel organosilicon compound and the method for the preparation thereof as well as a rubber composition comprising natural rubber or a diene-based synthetic rubber, a non-carbon inorganic filler and the organosilicon compound as a modifying agent of the properties of the rubber vulcanizates.
In comparison with a rubber composition formulated with a carbon black filler, as is well known, rubber compositions based on natural rubber or a diene-based synthetic rubber and formulated with a non-carbon inorganic filler are inferior in respect of the poor processibility of the unvulcanized rubber composition and the low mechanical properties of the vulcanizates thereof such as tensile strength, tear strength, elastic modulus, stress-strain characteristics, elastic resilience and anti-wearing resistance because the inorganic filler inherently has no affinity with the polymeric rubbery constituent and exhibits no reinforcing effect as a result of formation of the so-called carbon gel formed in the rubber compositions loaded with a carbon black filler.
Various proposals and attempts have been made hitherto to achieve an improvement of rubber compositions in this regard. One of the promising ways for the improvement in the prior art is to blend the rubber composition with an organosilicon compound having a polysulfide linkage in a molecule. For example, Japanese Patent Publication No. 51-28623 and Japanese Patent Kokai 52-83527 teach that the rubber composition may be admixed with an organosilicon compound represented by the general formula EQU (RO).sub.3 Si--R'--S.sub.x --R'--Si(OR).sub.3 or
[(X).sub.3 --y(R)ySi--R'.sub.n --Ar].sub.a [S.sub.x ].sub.b,
in which R is a monovalent hydrocarbon group, R' is a divalent hydrocarbon group, Ar is an aryl group, X is a hydrolyzable group, x is a positive integer of 2 to 6, y is zero, 1 or 2, n is zero or 1 and a and b are each a positive integer of at least 2 and at least 1 with the proviso that the ratio a:b is in the range from 0.4 to 2.
This method is indeed effective and substantial improvements can be obtained in the rubber vulcanizates of the compositions formulated with such an organosilicon compound relative to the tensile strength, tear strength, elastic modulus and stress-strain characteristics. The degree of improvements achieved by this method, however, is still insufficient in respect of the anti-wearing resistance and the mechanical properties of the vulcanizates at elevated temperatures. Accordingly, it has been eagerly desired to develop a novel additive compound capable of fully improving the rubber compositions without the above mentioned problems in the prior art additive compounds. The inventors have continued extensive investigations to discover an organosilicon compound having a polysulfide linkage and useful for the purpose and arrived at the completion of the present invention.